Double-sided FPCBs are widely used in various kinds of electronic devices. To make the FPCB, through holes are needed to be defined at opposite sides of the circuit substrate of the FPCB. Then, copper is plated in the through holes to form conductive vias that electrically connect the conductive wiring circuits on the opposite sides of the circuit substrate.
However, bubbles may occur during the plating of copper. The bubbles may burst when cured, which may decrease the quality of the FPCB.